1. Field of the Invention
This invention relates generally to a method for the assembly of free-standing films using a layer-by-layer process. More particularly, but not by way of limitation, the present invention relates to a method for producing ultra-thin membranes wherein the membrane is first assembled on a soluble or otherwise removable substrate. Upon completion of the membrane assembly, the substrate is dissolved in a suitable solvent or removed by other means leaving behind the free-standing membrane.
2. Background
Thin film technology, wherein inorganic particles with sizes on the order of 1–100 nm are arranged in layers to form a film, is being used presently for an increasingly large number of different technological applications, including, among other things, information storage systems, chemical and biological sensors, fiber-optical systems, magneto-optical and optical devices, pervaporation membranes, protective coatings and light emitting diodes. Current techniques for preparing such films include chemical vapor deposition (in which no discrete inorganic particles are involved), sol-gel technology (producing porous materials that can be sintered to get uniform films), or deposition from colloidal dispersions (spin-coating, dip-coating, Langmuir-Blodgett deposition, etc.).
Layer-by-layer assembly (LBL) is a method of thin film deposition which is often used for oppositely charged polymers or polymers otherwise having affinity and has recently been applied to the preparation of thin films of nanoparticles. Its simplicity and universality, complemented by the high quality films produced thereby, make the layer-by-layer process an attractive alternative to other thin film deposition techniques. LBL can be applied to a large variety of water-soluble compounds and is especially suitable for the production of stratified thin films in which layers of nanometer thickness are organized in a specific predetermined order. Such a process is described in U.S. patent application Ser. Nos. 60/151,511 and 09/492,951, which disclosures are incorporated herein by reference.
Typically, layer-by-layer films are assembled on a solid substrate material such as a glass slide or silicon wafer. Deposition of the film material onto the substrate is performed in a cyclic manner, made possible by the overcompensation of surface charge which often takes place when polyelectrolytes and other high molecular weight species are adsorbed on a solid-liquid interface. As used herein, a “high molecular weight” material refers to polymers, including proteins, nanoparticles, exfoliated clays and other organic and inorganic species, having a molecular weight greater than about 1000 atomic units. In one example of a layer-by-layer assembly process, after preparation of the substrate, a film is deposited on the substrate by repeating the process of: 1) immersion of the substrate in an aqueous solution of polyelectrolyte; 2) washing with neat solvent; 3) immersion in an aqueous dispersion of nanoparticles; and 4) final washing with neat solvent. This process is repeated as many times as necessary, depending on the number of layers required in order to obtain the specific properties of the desired material.
In the prior art, this process has been limited to applications wherein the substrate and the assembled film remain intact as a unitary structure. This limits the minimum thickness of a film and limits the application of films produced through this process to those tolerant of the substrate material. The present invention, however, provides a process wherein the assembled thin film may be separated from the substrate to form a free-standing thin film material overcoming these and other limitations.
Membranes are typically prepared by casting a solution of a polymer on a solid substrate. In this technique, the structure of the membrane is determined by the chemical structure of the casting material. Extensions of this generic method, including casting on an immiscible solvent or post factum surface grafting, can also yield asymmetrical membranes with chemically modified membrane surfaces.
It is thus an object of the present invention to provide a method for the assembly of free-standing thin film materials using a layer-by-layer process.
It is a further object of the present invention to provide a method for the assembly of free-standing thin film materials using a layer by layer process wherein the assembled thin film material exhibits structural properties which allow for manipulation of the assembled material.
It is another object of the present invention to provide a method for the assembly of free-standing thin film material which permits improved control over a membrane structure allowing for the production of stratified multifunction membranes.
It is yet another object of the present invention to provide a method for the assembly of free-standing thin film material which permits the incorporation of biological compounds into the membrane structure while retaining the biological activity of the compounds.
It is still another object of the present invention to provide a free standing thin film material wherein at least one layer of the material includes an inert structural stabilizing element, such as exfoliated montmorillonite clay platelets, or wherein cross-linking of layer by layer films is achieved, to improve the structural properties of the thin film material.